Silico-sodo-calcic glass sheet

ABSTRACT

The invention relates to a glass sheet, the composition of which is of the soda-lime-silica type and comprises the following constituents in contents varying within the weight limits defined below:
         Fe 2 O 3  (total iron) 0 to 0.02%; and   WO 3  0.1 to 2%.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of PCT/FR08/051686 filed Sep. 19,2008 and claims the benefit of FR 0757758 filed Sep. 21, 2007.

The present invention relates to a glass sheet having high transmissionproperties for visible and infrared radiation.

Although it is not limited to such an application, the invention will bemore particularly described with reference to glass sheets capable ofbeing obtained by the “float” process, consisting in pouring the moltenglass over a bath of molten metal (in particular tin).

In applications where the glass is used in the form of a glass sheetcovering photovoltaic cells or solar cells, it is essential that theglasses used have an extremely high transmission of visible and/orinfrared radiation, in particular greater than 90%, because the quantumefficiency of the cell may be strongly affected by an even very lowreduction in the transmission, by the glass, of visible or infraredradiation.

The transmission in the visible or infrared range is generally expressedin the form of a transmission factor that integrates, over a certainportion of the spectrum, the transmission for each wavelength takinginto account a given spectral distribution and optionally thesensitivity of the human eye. In order to quantify the transmission ofthe glass in the visible range, a light transmission factor is thusdefined, referred to as light transmission, often abbreviated to“T_(L),”, calculated between 380 and 780 mm and related to a glassthickness of 3.2 mm, while taking into consideration the illuminant D65as defined by the ISO/CIE 10526 standard and the CIE 1931 standardcolorimetric observer as defined by the ISO/CIE 10527 standard. Toquantify the transmission of the glass in the range encompassing thevisible and solar infrared (also known as “near infrared”) ranges, anenergy transmission factor is defined, known as “energy transmission”,abbreviated to “T_(E)”, calculated according to the ISO 9050 standardand related to a glass thickness of 3.2 mm.

It is known to decrease the total iron oxide content in the glass asmuch as possible in order to attain values of T_(L) and T_(E) greaterthan 90%. Iron oxide, present as an impurity in most of the natural rawmaterials used in the glass industry (sand, feldspar, limestone,dolomite, etc.), absorbs both in the visible and near ultraviolet range(absorption due to the ferric ion Fe³⁺) and especially in the visibleand near infrared range (absorption due to the ferrous ion Fe²⁺). Withordinary natural raw materials, the total weight content of iron oxideis around 0.1% (1000 ppm). Transmissions of more than 90% require,however, lowering the content of iron oxide to less than 0.02% or 200ppm, or even less than 0.01% (100 ppm), which makes it necessary tochoose particularly pure raw materials and increases the cost of thefinal product.

To increase the transmission of the glass even further, it is also knownto reduce the content of ferrous iron in favor of the content of ferriciron, therefore to oxidize the iron present in the glass. Thus glasseshaving the lowest possible “redox”, ideally zero or almost zero, aretargeted, the redox being defined as being the ratio of the weightcontent of FeO (ferrous iron) to the weight content of total iron oxide(expressed in the Fe₂O₃ form). This number may vary between 0 and 0.9,zero redoxes corresponding to a completely oxidized glass.

Various solutions have been proposed for oxidizing the iron oxide asmuch as possible. It is for example known from U.S. Pat. No. 6,844,280to add cerium oxide (CeO₂) to the glass. Cerium oxide is however capableof being the cause of the process known as “solarization”, in which thetransmission of the glass greatly decreases after absorption ofultraviolet radiation. It is also known to add to the glass antimonyoxide (Sb₂O₃) or arsenic oxide (As₂O₃), oxides that are conventionallyused as refining agents for glass and which have the particularity ofoxidizing the iron. The use of Sb₂O₃ is, for example, described inApplication US 2006/249199. These oxides have however proved to beincompatible with the float process of the glass. It would appear that,under the reducing conditions necessary for the non-oxidation of the tinbath, some of these oxides volatilize then condense on the glass sheetbeing formed, generating an undesirable haze.

The objective of the present invention is therefore to overcome theaforementioned drawbacks and to provide novel glass sheets for which thelight and energy transmission is very high, enabling them to be used inphotovoltaic cells.

For this purpose, one subject of the invention is a glass sheet, thechemical composition of which is of the soda-lime-silica type andcomprises the following constituents in contents varying within theweight limits defined below:

Fe₂O₃ (total iron)   0 to 0.02%; and WO₃ 0.1 to 2%.

Fe₂O₃ and WO₃ respectively represent the total contents of iron andtungsten oxides in the glass, regardless of the degree of oxidation oftheir respective ions.

The glass sheet is, in particular, capable of having been obtained by afloat process on a bath of molten tin.

The expression “composition of soda-lime-silica type” is understood tomean a composition comprising silica (SiO₂) as a forming oxide andsodium oxide (soda, Na₂O) and calcium oxide (lime, CaO). Thiscomposition preferably comprises the following constituents in contentsvarying within the weight limits defined below:

SiO₂ 60-75% Al₂O₃  0-10% B₂O₃  0-5%, preferably 0 CaO  5-15% MgO  0-10%Na₂O  5-20% K₂O  0-10% BaO  0-5%, preferably 0.

The presence of iron in a glass composition may result from the rawmaterials, as impurities, or from a deliberate addition aiming to colorthe glass. It is known that iron exists in the structure of the glass inthe form of ferric ions (Fe³⁺) and ferrous ions (Fe²⁺). The presence ofFe³⁺ ions gives the glass a very slight yellow coloration and makes itpossible to absorb ultraviolet radiation. The presence of Fe²⁺ ionsgives the glass a more pronounced blue-green coloration and induces anabsorption of infrared radiation. The increase in the content of iron inits two forms intensifies the absorption of radiation at the ends of thevisible spectrum, this effect taking place to the detriment of the lighttransmission.

In the present invention, the Fe₂O₃ (total iron) content is preferablyless than or equal to 0.015%, in particular 0.01%, in order to limit thelight and energy transmission.

Tungsten oxide has proved capable of oxidizing the iron, thus decreasingthe content of Fe²⁺ ions. This effect has never, to the knowledge of theinventors, been highlighted, and would appear to exist only in the caseof glasses that are very poor in iron oxide. Moreover, this oxide isperfectly compatible with the glass float process and the glass thusproduced does not exhibit solarization.

In order to maximize its effect, the WO₃ content is preferably greaterthan or equal to 0.2%, or even 0.3% and even 0.4% or 0.5%. It wouldappear, however, that the oxidation effect saturates beyond a certainvalue. Taking into account the cost of this oxide, its content ispreferably less than or equal to 0.9%, or even 0.8% and even 0.7%. Acontent of around 0.5% is preferred. Beyond 2% of WO₃, a phaseseparation is observed.

The WO₃ content is preferably between 0.2 and 1%, in particular between0.3 and 0.7% or between 0.3 and 0.5%. A content of around 0.35% ispreferred.

The redox, which is an indicator of the oxidation-reduction state of theglass, is preferably less than or equal to 0.1, or even 0.07 and even0.05, in order to maximize the transmission of the glass.

The glass sheet according to the invention preferably has, for athickness of 3.2 mm, a light transmission T_(L) of at least 91%, inparticular 91.1%, even 91.2% or 91.3% and even 91.4% or 91.5%.

Advantageously, it has, still for a thickness of 3.2 mm, an energytransmission T_(E) of at least 91%, in particular 91.5%, or even 91.2%or 91.3% and even 91.4% or 91.5%.

These values are close to the theoretical limit for a glass free ofantireflection treatments, since the reflection factor of one face ofsoda-lime-silica glass is around 4%, i.e. 8% when both faces are takeninto account. A soda-lime-silica glass free of antireflection treatmentscannot therefore have a transmission greater than 92%.

Within the context of the present invention, one particularly preferredcomposition comprises the following constituents in contents varyingwithin the weight limits defined below:

Fe₂O₃ (total iron) 0.010 to 0.015%; and WO₃  0.3 to 0.5%.

Such a composition makes it possible to obtain glasses of very low redox(0.07 or less), or even lower when the K₂O content of the matrix isgreater than or equal to 1.5%, in particular 2%, or even 3% and even 4%,as explained below.

It is recommended here that the soda-lime-silica glass composition maycomprise, besides the inevitable impurities contained, in particular, inthe raw materials, a small proportion (up to 1%) of other constituents,for example agents that aid the melting or refining of the glass (SO₃,Cl, etc.), or else elements that originate from the dissolution of therefractories that are used in the construction of the furnaces (forexample, ZrO₂). For reasons already mentioned, the composition accordingto the invention preferably does not comprise oxides such as Sb₂O₃,As₂O₃ or CeO₂. Preferably, the MoO₃ content is zero.

The composition of the glass sheet according to the invention preferablydoes not comprise any agent that absorbs visible or infrared radiation(especially for a wavelength between 380 and 1000 nm) other than thosealready cited. In particular, the composition according to the inventionpreferably does not contain agents chosen from the following agents, orany of the following agents: transition element oxides such as CoO, CuO,Cr₂O₃, MnO₂, rare-earth oxides such as CeO₂, La₂O₃, Nd₂O₃, or elsecoloring agents in the elemental state such as Se, Ag, Cu. These agentsvery often have a very powerful undesirable coloring effect, which ismanifested at very low contents, sometimes of around a few ppm or less(1 ppm=0.0001%). Their presence thus very strongly decreases thetransmission of the glass. For certain applications, in particular infurniture, it has however possible to add a very small amount of acoloring oxide, in particular cobalt oxide in a content less than 1 ppm,to give a slight visible coloration at the edge of the glass.

In the glass sheets according to the invention, silica is generally keptwithin narrow limits for the following reasons. Above 75%, the viscosityof the glass and its ability to denitrify increase greatly, which makesit more difficult to melt and flow on the bath of molten tin. Below 60%,in particular 64%, the hydrolytic resistance of the glass decreasesrapidly.

Alumina, Al₂O₃, plays a particularly important role in the hydrolyticresistance of the glass. When the glass according to the invention isintended to be used in hot and humid environments, the alumina contentis preferably greater than or equal to 1%.

The alkali metal oxides Na₂O and K₂O facilitate the melting of the glassand allow its viscosity to be adjusted at high temperatures so as tokeep it close to that of a standard glass. K₂O may be used up to 10%, asthe problem of the high cost of the composition is faced above thiscontent. Moreover, the increase in the percentage of K₂O can beaccomplished, essentially, only to the detriment of Na₂O, whichcontributes to increasing the viscosity. The sum of the Na₂O and K₂Ocontents, expressed as percentages by weight, is preferably equal to orgreater than 10% and advantageously less than 20%. If the sum of thesecontents is greater than 20% or if the Na₂O content is greater than 18%,the hydrolytic resistance is greatly reduced. The glasses according tothe invention are preferably free of lithium oxide Li₂O due to its highcost.

It is apparent to the inventors that the increase in the K₂O content(especially to the detriment of the Na₂O content) would make it possibleto obtain even higher transmissions. This phenomenon could be due to amodification of the chemical environment of the iron atoms.

Therefore, the K₂O content is preferably greater than or equal to 1.5%,or even 2%, in particular 3%, and even 4% or 5%. For reasons mainlylinked to the cost, the K₂O content is preferably less than or equal to8%, or even 7%. A content between 3% and 5% has proved particularlyadvantageous.

Alkaline-earth metal oxides make it possible to adapt the viscosity ofthe glass to the production conditions.

MgO may be used up to around 10% and its omission may be at least partlycompensated for by an increase in the Na₂O and/or SiO₂ content.Preferably, the MgO content is less than 5%. Low MgO contentsfurthermore make it possible to reduce the number of raw materialsneeded for melting the glass.

BaO has a much smaller influence than CaO and MgO on the viscosity ofthe glass and the increase in its content is essentially accomplished tothe detriment of the alkali metal oxides, of MgO and especially of CaO.Any increase in BaO contributes to increasing the viscosity of the glassat low temperatures. Preferably, the glasses according to the inventionare free of BaO and also of strontium oxide (SrO), these elements havinga high cost.

The glass composition according to the invention is capable of beingmelted under the conditions for producing glass intended for formingflat glass by the drawing, rolling or, preferably, floating techniques.Melting generally takes place in flame-fired furnaces, optionallyprovided with electrodes for heating the glass in the bulk, by passingan electric current between the two electrodes. To facilitate themelting operation, and especially to make the latter mechanicallyadvantageous, the glass composition advantageously has a temperaturecorresponding to a viscosity η such that log η=2 that is less than 1500°C. Also preferably, the temperature corresponding to the viscosity ηsuch that log η=3.5 (denoted by T(log η=3.5)) and the liquidustemperature (denoted by T_(liq)) satisfy the equation:T(log η=3.5)−T _(liq)>20° C.and better still:T(log η=3.5)−T _(liq)>50° C.

In order to further improve the light and energy transmission of theglass, the glass sheet according to the invention may be coated on atleast one of its faces with an antireflection coating. This coating maycomprise one layer (for example, based on porous silica having a lowrefractive index) or several layers: in the latter case a stack oflayers based on dielectric alternating layers of low and high refractiveindices and finishing with a layer having a low refractive index ispreferred. It may especially be a stack described in Application WO01/94989 or WO 2007/077373.

Still in order to increase the light and energy transmission, thesurface of the glass sheet may be textured, for example having patterns(in particular pyramid-shaped patterns), as described in Applications WO03/046617, WO 2006/134300, WO 2006/134301 or else WO 2007/015017.

Another subject of the invention is the use of the glass sheet accordingto the invention in photovoltaic cells, solar cells, flat or parabolicmirrors for concentrating solar energy, or else diffusers forbacklighting display screens of the LCD (liquid crystal display) type.The glass sheet according to the invention may also be used for interiorapplications (partitions, furniture, etc.) or in electrical goods(refrigerator shelves, etc.). It may also be used in displays or flatlamps based on organic light-emitting diodes.

Generally, another subject of the invention is a photovoltaic cell, asolar cell or a flat or parabolic mirror for concentrating solar energy,or else a diffuser for backlighting display screens of the LCD typecomprising at least one glass sheet of which the light transmissionT_(L) is at least 91%, or even 91.5% and/or of which the energytransmission T_(E) is at least 91%, or even 91.4% (in both cases in theabsence of any antireflection treatment) for a thickness of 3.2 mm,independently of the composition of the glass. The glass sheet ispreferably capable of having been obtained by a float process on a bathof molten tin. Specifically, it appears that the invention has made itpossible, for the first time, to obtain such performances.

The present invention will be better understood on reading the detaileddescription below of nonlimiting exemplary embodiments illustrated bytable 1.

Indicated in these examples are the values of the following opticalproperties calculated for a glass thickness of 3.2 mm from experimentalspectra:

-   -   the energy transmission (T_(E)) calculated according to the ISO        9050 standard;    -   the overall light transmission factor (T_(L)), calculated        between 380 and 780 mm, taking into consideration the illuminant        D65 as defined by the ISO/CIE 10526 standard and the CIE 1931        standard colorimetric observer as defined by the ISO/CIE 10527        standard.

Also indicated in table 1 are the weight contents of potassium, iron andtungsten oxides, measured by chemical analysis.

The compositions appearing in table 1 are produced from a matrix whichcomprises the following oxides, the contents of which are expressed inpercentages by weight:

SiO₂ 71.0%  Al₂O₃ 0.8% CaO 9.5% MgO 4.0% Na₂O 13.75% 

Added to this matrix is a given amount of K₂O, in substitution for Na₂O.

TABLE 1 C1 1 2 3 4 5 K₂O (%) 0.35 0.35 2.0 0.35 5.0 3.5 Fe₂O₃ (%) 0.010.01 0.01 0.01 0.01 0.01 WO₃ (%) 0 0.2 0.2 0.5 0.5 0.35 T_(L) (%) 90.791.2 91.4 91.3 91.5 91.5 T_(E) (%) 90.5 90.9 91.1 91.0 91.4 91.4

The composition C1 is a comparative example, that does not comprisetungsten oxide. The introduction of this oxide makes it possible tosubstantially increase the light and energy transmissions of the glass,up to values greater than or equal to 91%. The use, in examples 2, 4 and5, of a matrix containing more than 1.5% of K₂O, even makes it possibleto attain values of 91.5%.

Example 5 is particularly advantageous since it has very hightransmission values for a moderate content of WO₃ and K₂O.

None of the glasses obtained exhibits solarization after an acceleratedtest of 100 hours under ultraviolet radiation.

1. A glass sheet comprising a soda-lime-silica composition in weightpercentages: B₂O₃ 0 to 5% Fe₂O₃ (total iron) 0 to 0.02%; and WO₃ 0.1 to2%.


2. The glass sheet as claimed in claim 1, which comprises in weightpercentages: SiO₂ 60-75%  Al₂O₃ 0-10% B₂O₃ 0%,  CaO 5-15% MgO 0-10% Na₂O5-20% K₂O 0-10% BaO 0-5%. 


3. The glass sheet as claimed in claim 1, wherein the Fe₂O₃ (total iron)content is less than or equal to 0.015%.
 4. The glass sheet as claimedin claim 1, wherein the WO₃ content is between 0.3 and 0.7%.
 5. Theglass sheet as claimed in claim 1, wherein the redox is less than orequal to 0.1.
 6. The glass sheet as claimed in claim 1, having, for athickness of 3.2 mm, a light transmission T_(L) of at least 91%.
 7. Theglass sheet as claimed in claim 1, having, for a thickness of 3.2 mm, anenergy transmission T_(E) of at least 91%.
 8. The glass sheet as claimedin claim 1, such that the K₂O content is greater than or equal to 1.5%.9. The glass sheet as claimed in claim 1, the composition of which doesnot contain any other agent that absorbs visible and infrared radiation.10. The glass sheet as claimed in claim 9, the composition of which doesnot contain agents selected from the group consisting of CoO, CuO, Cr₂O₃MnO₂, CeO₂, La₂O₃, Nd₂O₃, Se, Ag, and Cu.
 11. The glass sheet as claimedin claim 1, the composition of which does not contain the oxides Sb₂O₃,As₂O₃ or CeO₂.
 12. A diffuser comprising the glass sheet as claimed inclaim
 1. 13. A method of backlighting the display screen of an LCDcomprising backlighting the display screen of an LCD with the diffuseras defined in claim
 12. 14. A photovoltaic cell comprising a glass sheetcomprising in weight percentages a soda-lime-silica composition, Fe₂O₃(total iron)   0 to 0.02%; and WO₃ 0.1 to 2%.


15. A solar cell comprising a glass sheet comprising in weightpercentages a soda-lime-silica composition, Fe₂O₃ (total iron)   0 to0.02%; and WO₃ 0.1 to 2%.


16. A flat or parabolic mirror that concentrates solar energy comprisinga glass sheet comprising in weight percentages a soda-lime-silicacomposition, Fe₂O₃ (total iron)   0 to 0.02%; and WO₃ 0.1 to 2%.